NullFree* annotations

[eparovyshnaya]

@NullFreeZone

annotation would be a nice way to publicly manifest intolerance to null in all it's forms and origins.

Implemented as with source retention and available for package/type/field/constructor/method , it can beget compile time errors for any null literal usages in the annotated code.

@NullFreeContract

as a weaker variant, would declare (and optionally contribute to JavaDoc) that

• none of method s in scope can either accept null as a parameter or return it as a result
• nont of field s in scope can be assigned to null